MULLITE-ALUMINUM TITANATE DIESEL EXHAUST FILTER

Description

BACKGROUND OF THE INVENTION

[0001] The invention relates to a diesel exhaust filter. Specifically the invention relates to a mullite aluminum titanate diesel exhaust filter and a method of making the same.

[0002] In the industry cordierite (2MgO·2Al₂O₃·5SiO₂) has been the cost-effective material of choice for diesel exhaust aftertreatment applications, such as wall-flow filters, due to its combination of good thermal shock resistance, filtration efficiency, and durability under most operating conditions. However, under certain circumstances cordierite filters are susceptible to damage and have even catastrophically failed. Occasional thermal runaway occurs during uncontrolled regenerations resulting in localized cordierite melting. Another factor contributing to failure occurs when metal impurities from the engine oil, catalytic additives or corroded metal from the exhaust port liners are introduced into the filter during operation. Typically, at temperatures exceeding 1300°C, these metals form oxides which react with the cordierite structure. Evidence of the failed material are usually small holes on the filter where the metal initially deposits and reacts causing corrosion and melting of the material.

[0003] Recently, silicon carbide (SiC) wall-flow filters have been made commercially available for diesel exhaust filtration. However, SiC filters are costly to manufacture, and must be segmented due to an inherent high coefficient of thermal expansion (CTE) and poor thermal shock resistance.

[0004] Aluminum titanates, in particular mullite aluminum titanate, have been proposed for usage as filters, in particular, as diesel particulate filters in U.S. Pat. Nos. 4,483,944, 4,767,731, 4,855,265, 5,290,739 and EP 0 036 462 and EP 0 873 775. Such bodies, however, are expected to have low permeability, and consequently high pressure drop or back pressure against the engine, due to low porosity or fine pore size; or in other cases, to have porosities and pore sizes so large that the bodies would be expected to exhibit poor filtration efficiency in removal of particles from a gas stream.

[0005] US-A-4 329 162 teaches a diesel exhaust particle filter comprising a plugged honeycomb filter body made of porous ceramic material. The prior art document "LACHMANN I.M. ET AL.: "High-Temperature Monolithic Supports for Automobile Exhaust Catalysts", CERAM. ENG. SCI. PROC. Vol. 2, no.5-6, 1981, pages 337-351 teaches examples of honeycomb filter bodies. The MCB-13 composition has a 7 micron MPS. The firing temperature for MCB-13 is low. US-A-5 288 672 teaches firing at between 1250-1500°C and the use of very fine (< 2 micron) powders. US-A-4 483 944 teaches when a low firing temperature of 1400°C is used, a porosity of 30-45% may be obtained, and when a higher temperature of 1500°C is used, the resultant porosity is reduced to 8-26%.

[0006] A need therefore exists to have a mullite aluminum titanate diesel particulate filter with higher permeability than has been achieved up to now, while at the same time with useful filtration efficiency, low thermal expansion, high heat capacity, high thermal shock resistance, high mechanical strength, and increased durability during thermal cycling.

SUMMARY OF INVENTION

[0007] A key aspect of the present invention is a mullite-aluminum titanate ceramic article of high interconnected porosity and large median pore size, in combination with high permeability which is capable of employment as a filter for diesel exhaust for its low pressure drop and low back pressure against the engine. Concurrently, the inventive article exhibits low thermal expansion, high heat capacity, high thermal shock resistance and high mechanical strength along with useful filtration efficiency and increased durability during thermal cycling.

[0008] In particular the present invention relates to a diesel exhaust particulate filter comprising a plugged, wall-flow honeycomb filter body composed of porous ceramic material and having a plurality of parallel end-plugged cell channels traversing the body from a frontal inlet end to an outlet end thereof. The porous ceramic contains, expressed in terms of weight percent of the total body, of 60-90%, preferably 70-80%, most preferably 70% iron-aluminum titanate solid solution having a stoichiometry of Al_2(1-X)Fe_2xTiO₅, where x is 0-0.1, and 10-40%, preferably 20-30%, most preferably 30% mullite (3Al₂O₃·2SiO₂), and consists essentially, expressed in terms of weigh percent on the oxide basis, of 3 to 15%, preferably 6 to 12% SiO₂, 55 to 65%, preferably 57 to 61% Al₂O₃,22 to 40%, preferably 26 to 35% TiO₂, and 0 to 10%, preferably 0.5 to 5% Fe₂O₃.

[0009] Properties of the inventive structures generally include a porosity of 35% to 55%, preferably 40 to 50% by volume; a median pore size of 8 to 20 micrometers, preferably 10 to 15 micrometers; a permeability of at least 0.30 x 10^-12 m², preferably 0.33 to 1.00 x 10^-12 m² when fired to a temperature of between 1650° to 1700°C; an increment in a linear dimension of less than 1% after the filter has been exposed one hundred times to thermal cycling at a temperature of 200° to 1100°C in a time of 7 minutes, and cooling from 1100° to 200°C in a time of 7 minutes; a CTE, as measured from room temperature to 1000°C of 5 to 15 x 10^-7/°C, preferably 5 to 10 x 10^-7/°C; a heat capacity of at least 3.2 J cm^-3K^-1, preferably at least 3.9 J cm^-3K^-1 at 500°C; and, a four-point modulus of rupture as measured on a solid rod of circular cross section of at least 34 bar (500 pounds per square inch), preferably 55 bar (800 pounds per square inch).

[0010] Another aspect of the invention is a method of making the diesel particulate filter which includes forming a mixture of powdered raw materials selected to obtain a ceramic article consisting essentially, expressed in terms of weigh percent on the oxide basis, of 3 to 15% SiO₂, 55 to 65% Al₂O₃, 22 to 40% TiO₂, and 0 to 10% Fe₂O₃; shaping the mixture into a green ceramic article, preferably by extrusion into a honeycomb having an inlet and outlet end and a multiplicity of cells extending from inlet end to outlet end, the cells having porous walls; and, firing the green ceramic article to a temperature of between 1650° to 1700°C for a duration of time sufficient to produce a mullite-aluminum titanate.

BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 is a schematic of a diesel particulate filter of the present inveniton.

[0012] FIG. 2 is a SEM photograph at a 500X magnification representative of the microstructure of the inventive structures.

[0013] FIG. 3 is a SEM photograph at a 500X magnification representative of the microstructure of the inventive structures after thermal cycling tests.

DETAILED DESCRIPTION OF INVENTION

[0014] A wall-flow filter for diesel exhaust purification according to the present invention is shown in FIG. 1. The filter structure 10 is composed of a honeycomb body 15 having an inlet end 12, an outlet end 13, and a plurality of cells 11 extending in parallel between the inlet end 12 and the outlet end 13. The cells 11 have porous walls 14 and contain plugs 61 at one end only, either at the inlet end 12 or outlet end 13. Plugs 61 are arranged in a checkered pattern such that exhaust which enters the filter structure 10 through open cells 11 at inlet end 12 is forced to pass through porous walls 14 in order to exit outlet end 13.

[0015] The honeycomb body 15 of filter structure 10 comprises a porous ceramic material consisting essentially of two crystal phases; viz., iron-aluminum titanate solid solution having a stoichiometry of Al_2(1-X)Fe_2XTiO₅, where x is 0-0.1 and mullite having a stoichiometry of 3Al₂O₃·2SiO₂. A representative SEM photograph at a 500X magnification of the microstructure is shown in FIG. 2 in which the aluminum titanate phase is represented by numeral 20 and the mullite phase is represented by numeral 22.

[0016] As known in the art (see U.S. Pat. No. 4,855,265) aluminum titanate tends to show structural instability at high temperatures between 800° and 1300°C. Isomorphic substitution of iron (Fe) for the aluminum (Al) within the aluminum titanate phase is found to improve the stability of the material against decomposition. Therefore, iron oxide (Fe₂O₃) may be added in the present invention.

[0017] Accordingly, the composition of the articles of the present invention, consists essentially, expressed in weight percent based on oxides, of 3 to 15 %, preferably 6 to 12 % SiO₂; 55 to 65 %, preferably 57 to 61 % Al₂O₃; 22 to 40 %, preferably 26 to 35 % TiO₂; 1 to 10 %, preferably 0.5 to 5 % Fe₂O₃. Traces of a glassy phase may be present, along with traces of unreacted of alumina (Al₂O₃) and titania (TiO₂) if the structures are fired below 1650°C.

[0018] The composition may also expressed in terms of weight percent of the total body, of 60-90%, preferably 70-80%, most preferably 70% iron-aluminum titanate solid solution having a stoichiometry of Al_2(1-x)Fe_2xTiO₅, where x is 0-0.1, and 10-40%, preferably 20-30%, most preferably 30% mullite (3Al₂O₃·2SiO₂).

[0019] Aluminum titanate has a high melting point of 1860°C, and a highly anisotropic thermal expansion (i.e., widely differing expansions along the crystallographic axes) of about -29 x 10^-7/°C along the a-axis, about 103 x 10^-7/°C along the b-axis, and about 201 x 10^-7/°C along the c-axis. The extreme anisotropy causes the formation of internal stress within large aluminum titanate grains. This results in severe microcracking as the material cools which is attributed to its low coefficient of thermal expansion. Larger grain growth and subsequent microcracking, however, produce a low strength material.

[0020] The addition of mullite as a second phase, serves to increase the strength (modulus of rupture) and balance the CTE. Specifically, mullite improves the overall strength of the final body by containment of grain growth and microcrack propagation. The resulting mullite-aluminum titanate article exhibits microcrocracking and a very low CTE, leading to high thermal shock resistance. Microcracked bodies tend to bias the CTE towards the most negative CTE component because the opening of microcracks on cooling accommodates the normal positive components.

[0021] The inventive filters have desired properties suitable for low pressure drop and back pressure against the engine, and optimum efficiency. These properties include high interconnected porosity, large median pore size, and narrow pore size distribution, in combination yielding high permeability, while at the same time still maintaining a low coefficient of thermal expansion, high heat capacity, high thermal shock resistance and high mechanical strength, coupled with high durability during repeated thermal cycling. The desired properties are obtained by a careful selection of raw materials with specific particle size restrictions, and exposing a structure formed therefrom to specific heating requirements, as will become apparent from a description herein below.

[0022] The open porosity, as measured by mercury porosimetry, is about 35 % to 55 % by volume, and preferably about 40 % to 50 % by volume. The median pore size, also as measured by mercury porosimetry, is about 8 to 20 micrometers, and preferably about 10 to 15 micrometers.

[0023] The permeability of air under an applied pressure through the structure in the present invention was measured on the fired ribbon or pieces of cell wall using a Perm Automated Porometer® Version 6.0 (Porous Materials, Inc., Ithaca. NY). The value of the permeability is obtained as follows. A piece of fired cell wall is mounted with epoxy onto a disc-shaped sample holder which has a circular opening. The epoxy is applied around the perimeter of the opening such that the sample covers the opening and such that no air can pass through the opening without passing through the sample, and such that the area of the sample through which the air passes is very nearly the same as the area of the circular opening of the sample holder. The sample is then placed in the chamber of the automated porometer and a differential air pressure is applied across the sample. The volumetric air flow on the outlet end of the sample is measured as a function of the pressure applied to the inlet face of the sample. The specific permeability, k, is then computed from the following relation:


where η is the viscosity of air at room temperature in units of megapascal seconds, L is the thickness of the sample in units of meters, Q is the uniaxial volume flow rate of air through the sample in cubic meters per second, A is the area through which air is allowed to pass through the sample, approximately equal to the area of the opening of the sample holder, in units of square meters, and P is the differential pressure across the thickness of the sample in units of megapascals. The permeability, is thus expressed in units of square meters, 10^-12 m². For low pressure drop and low back pressure against the engine, accordingly the permeability is at least about 0.30 to 1.00 x 10^-12 m², preferably 0.33 to 1.00 x 10^-12 m².

[0024] While the inventive structures have a high interconnected porosity and a large median pore size, there is still maintained a low coefficient of thermal expansion, high heat capacity, high thermal shock resistance and high mechanical strength. The coefficient of thermal expansion (CTE), as measured by dilatometry, from room temperature (22°C) to 1000°C is between 5 and 15 x 10^-7 /°C, preferably between 5 and 10 x 10^-7/°C. The structures exhibit a four-point modulus of rupture as measured on a solid rod of circular cross section of at least 34 bar (500 pounds per square inch (psi)) and preferably at least 55 bar (800 psi). In experiments after thermal cycling at 1000°C for fifty cycles, the strength loss is less than 15%.

[0025] in further experiments carried out at high temperatures between 600°C and 1100°C, the heat absorbed by the inventive structures per unit volume is estimated to be about 25% higher than current commercially available cordierite diesel particulate filters. This indicates a high heat capacity which means that the temperature in the filter during an uncontrolled regeneration will be lower for structures made according to the present invention than for cordierite, for the same part design (porosity, cell density, and wall thickness). The heat capacity as measured at 500°C is greater than 3.2 J cm^-3 K^-1, and preferably at least 3.9 J cm^-3 K^-1. The highest exotherm observed at the highest level of soot loading (20 g/L) was no greater than about 1010°C. This is remarkable considering that exotherms as high as 1400°C have been observed with current cordierite diesel particulate filters.

[0026] A further advantage of the inventive structures is increased durability during thermal cycling. In experiments performed, a heat curve in a thermal cyclic test consists of a temperature rising from 200°C to 1100°C in 7 minutes and cooling from 1100°C to 200°C in 7 minutes. After one hundred times of exposure this heat cycle, the growth observed was less than 1 %. The size of the samples tested were measured with a super micrometer before and after testing. FIG. 3 is an SEM photograph at a 500X magnification of the same sample depicted in FIG. 2 after undergoing thermal cycling between 200°C to 1100°C at 100 times. The mullite phase 22 remains intact.

[0027] A still further advantage of the inventive structures is increased ash resistance. Metal oxide "ash" particles exist in diesel exhaust and are not combustible; therefore, cannot be easily removed during regeneration, like carbon soot. If filter temperatures during regenerations reach sufficiently high values (i.e., > 1200°C) these ash particles may sinter to the filter material and undergo chemical reactions.

[0028] In the present invention ash resistance was evaluated using diesel ash and particulate Fe₂O₃. The experiments involved obtaining samples from a honeycomb structure of approximately 30 mm x 20 mm x 5 mm. A thin layer of ash or Fe₂O₃ of known weight per unit area was applied to the surface of the samples. The typical loading was about 0.011 grams/cm² in all cases. The loading was chosen to simulate the amount of ash that would accumulate on a standard 14.38 cm (5.66-inch) diameter, 15.24 cm (6-inch) long filter with 31 cells/cm² (200 cells/inch²) and a wall thickness of 0.046 cm (0.018 inches) after approximately 100,000 km of driving.

[0029] The ash/Fe₂O₃-coated samples were placed into furnaces that had been pre-heated to a desired temperature, held in the furnace for one hour, then removed and allowed to cool to room temperature. The tests were conducted at 1000°, 1100°, 1300° and 1400°C. Samples were then visually and optically examined for extent of reaction under a stereo-microscope. The one-hour hold time at temperature is believed to be on the order of the cumulative time that the filter will experience temperatures >1000°C during its lifetime. This is based on 600 regenerations in the filter's lifetime, of which about 10% may be uncontrolled regenerations that might create temperatures in excess of 1000°C for about one to two minutes per uncontrolled regeneration.

[0030] The structures were found to be highly resistant to both diesel ash and particulate Fe₂O₃. No sintering or reactions were observed up to 1400°C. Also, as previously described the high heat capacity associated with the inventive materials, reduces the magnitude of the exotherm or temperature increase in the filter during regeneration. Lower exotherms result in less sintering of the metal oxide ash, and less reaction of the ash with the filter, thus increasing the lifetime of the filter.

[0031] To obtain mullite aluminum titanate diesel particulate filters as have been described, it has been discovered that a careful method of fabrication must be followed. This method involves the careful selection of specific raw materials having tailored particle size distributions.

[0032] The raw materials include (1) oxide sources that react to form the aluminum titanate and mullite phases, and/or (2) partially or completely pre-reacted aluminum titanate and mullite powders. All starting raw materials are chosen to limit the presence of alkali elements in the composition. Coarse median particle size of the raw materials is necessary for a sufficiently large median pore size and high permeability. Median particle size is the median particle diameter in micrometers based upon a volumetric distribution. It is determined by a particle size analyzer employing the laser diffraction technique, known to those of skill in the art. For partially or completely pre-reacted aluminum and mullite powder, the median particle size of the batched constituents is at least about-15-micrometers, preferably at least-about 20 micrometers, and more preferably at least about 35 micrometers. Of preference in terms of raw materials, however, are oxide sources that react to form the aluminum titanate and mullite phases. Such advantageous raw materials include silica (SiO₂), alumina (Al₂O₃), and titania (TiO₂).

[0033] The alumina-forming source is a powder which, when heated to a sufficiently high temperature in the absence of other raw materials, yields substantially pure aluminum oxide. Suitable alumina-forming sources include alpha-alumina, a transition alumina such as gamma-alumina or rho-alumina, boehmite, aluminum hydroxide, and mixtures thereof. The alumina-forming source is chosen to be of tabular morphology with a particle size sufficiently large to obtain the described porosity, while at the same time develop microcracking in the final structure. Accordingly, the particle size of the alumina source is between about 5 to 25 micrometers, and preferably between about 10 to 15 micrometers. A preferred source of alumina is Alutab® available from Alcan Chemical Limited, Oxon, England.

[0034] The preferred silica-forming source in the present invention is quartz, such as Silverbond® 200 available from Unimin Corporation of Canaan, CT. It has been discovered that the use of quartz plays an important role in the formation of porosity as described and the unique microstructure of the inventive structures. Other silica-forming sources include cristobalite, non-crystalline silica such as fused silica or sol-gel silica, zeolite, and diatomaceous silica. The use of kaolin, as a source of silica, is discouraged in the present invention. This is because kaolin negatively impacts the porosity and microstructure, as well as promoting poor cycling stability. The median particle size of the silica source is between about 5 to 20 micrometers, and preferably between about 8 to 15 micrometers.

[0035] The titania-forming source is preferably, but not limited to, rutile, such as Titanox Kronos® 3020, available commercially from Kronos, Inc. of Cranbury, NJ. The median particle size of the titania-forming source is important to avoid entrapment of unreacted oxide by the rapidly growing nuclei in the structure. According the median particle size is between about 4 to 20 micrometers, and preferably between about 8 to 16 micrometers.

[0036] The preferred source for iron oxide (Fe₂O₃) is iron titanium oxide having a particle size of between about 5 to 30 micrometers, and preferably between about 14 to 26 micrometers. A most preferred example is llmenite available commercially from Atlantic Equipment Engineers of Bergenfield, NJ. llmenite is preferred because of the propensity for liquid formation at 1100°C which improves the reaction to form the iron stabilized aluminum titanate phase. In the present invention isormorphic substitution of iron for the aluminum within the aluminum titanate phase is limited to a range between about 0-10 mole%, preferably about 8 mole%. This would account for an iron oxide addition of about 0% to 5.9% by weight, preferably up to 4.8% by weight.

[0037] In the process of the present invention, an advantage resides in the absence of a pore forming agent, such as graphite, in the raw material mixture. A pore former is a fugitive particulate material which evaporates or undergoes vaporization by combustion during drying or heating of the green body to obtain a desired, usually larger porosity and/or coarse median pore diameter.

[0038] A number of benefits flow in the absence of a pore former, including reduced firing times, reduced variability in physical properties, such as back pressure and CTE, and reduced gradients in these properties between the inner and outer portions within the structure. That being said, a pore former may be optionally included into the raw materials batch if so desired.

[0039] Raw materials are blended together with organic constituents that may include plasticizers, lubricants, binders, and solvents. Methyl cellulose is used as an organic binder which is later burned-off during firing. Oleic acid is preferably used as a dispersant. Sodium stearate is not recommended due to contamination from the sodium. Diglycol stearate may be used but typically leads to poor knitting during extrusion, resulting in crack formation during firing. If diglycol stearate is used, addition of approximately 1-2% by weight polyethylene glycol is required to serve as a lubricant. Water may also optionally be added as a solvent.

[0040] The mixture is then shaped into a green honeycomb body, such as by extrusion, optionally dried, and fired at a temperature and for a time sufficient to form the final product structure. The firing may be conduced in an electrically heated furnace or gas kiln at rates ranging from 10-200°C/hr over various temperature intervals to a maximum temperature of 1650-1700°C over a period of 25-30 hours, and cooled by shutting off power to the furnace. In a preferred embodiment the firing schedule is as follows: heating from room temperature (22-25°C) to 1500°-1600°C over a period of about 8 to 10 hours, with a hold at top temperature of about 4 to 8 hours, followed by heating over a period of about 2 to 3 hours to a top temperature of 1650° to 1700°C, with a hold thereat of about 30 to 60 minutes, followed by a ramp down to a temperature of about 1575° to 1600°C over a period of about 1 hour, with a hold thereat of about 4 to 5 hours. The short excursion to top temperature is necessary for phase and microcracking development, low shrinkage (<5%), along with large interconnected open porosity, large median pore diameter and high permeability.

[0041] A representative example of a diesel particulate filter provided in accordance with the invention, and the performance advantages resulting therefrom, may be better understood from the following illustrative example, which is not intended to be limiting.

Example

[0042] About 1 % by weight (of the total ceramic materials therein) oleic acid lubricant/extrusion aid was mixed with about 8.36% by weight, Silverbond® 200 silica for about 10 to 15 minutes. The remainder of the batch ceramic materials consisting essentially of: 58.46% by weight Alutab® alumina, 30.14% by weight Rutile Kronos® 3020 titania, and 3.13% by weight Ilmenite® iron titanium oxide were dry blended with (as weigh percent of the total ceramic materials therein) 4 % methyl cellulose plasticizer/binder. The mixture was plasticized with water in a mix-muller, and further plasticized and deaired by pre-extrusion into spaghetti-like masses. Thereafter the fully plasticized and compacted batch was extruded into honeycomb green shapes, dried for about 15 minutes in a dielectric oven, transferred to a regular oven for drying overnight, and then fired according to preferred schedule described herein above.

[0043] Table I below reports the constituents of the example in terms of weight percent on the oxide basis and also the phase assemblage. Table ll reports the physical properties measured thereon. The large density of 3 g/cm³ is indicative of a high heat capacity body. The high permeability at 0.33 x 10^-12 m² supports a low pressure drop and consequently a low back pressure against the engine. No deformation in the structure was observed below 1700°C.

[0044] It should be understood that while the present invention has been described in detail with respect to certain illustrative and specific embodiments thereof, it should not be considered limited to such but may be used in other ways without departing from the scope of the appended claims.

TABLE 1

Phase Assemblage	Weight percent of total body
Iron-Aluminum Titanate Solid Solution	70%
Mullite	30%

Oxide Composition	Weight percent of oxides
Al2O3	58.46%
TiO2	30.14%
SiO2	8.36%
Fe2O3	1.28%

TABLE 2

Property	Inventive Example
Porosity (% volume)	41.0
Median pore size (micrometers)	11.0
Permeability (10-12/m2)	0.33
CTE (22°C -1000°C) 10-7/°C	13.0
CTE(22°C -800°C) 10-7/°C	7.0
Density (g/ml)	3.37
Melting point (°C)	1700
Shrinkage (%)	2.0
Thermal Cycling Growth (100 cycles at 1100°C)	< 1 %

Claims

1. A diesel exhaust particulate filter comprising a plugged, wall-flow honeycomb filter body composed of porous ceramic material and comprising a plurality of parallel end-plugged cell channels traversing the body from a frontal inlet end to an outlet end thereof, wherein:

the ceramic contains, expressed in terms of weight percent of the total body, of 60-90% iron-aluminum titanate solid solution having a stoichiometry of Al_2(1-X) Fe_2xTiO₅, where x is 0-0.1, and 10-40% mullite (3Al₂O₃·2SiO₂), and consists essentially, expressed in terms of weigh percent on the oxide basis, of 3 to 15% SiO₂, 55 to 65% Al₂O₃, 22 to 40% TiO₂, and 0 to 10% Fe₂O₃; and,

the filter has a porosity of 35% to 55% by volume, a median pore size of 8 to 20 micrometers, a permeability of at least 0.30 x 10^-12 m² when fired to a temperature of between 1650° to 1700°C, and, an increment in a linear dimension of less than 1 % after the filter has been exposed one hundred times to thermal cycling at a temperature of 200° to 1100°C in a time of 7 minutes, and cooling from 1100° to 200°C in a time of 7 minutes.

2. The diesel exhaust particulate filter of claim 1 wherein:

the ceramic contains, expressed in terms of weight percent of the total body, of 70-80% iron-aluminum titanate solid solution having a stoichiometry of Al_2(1-X)Fe_2XTiO₅, where x is 0-0.1, and 20-30% mullite (3Al₂O₃·2SiO₂), and consists essentially, expressed in terms of weigh percent on the oxide basis, of 6 to 12% SiO₂, 57 to 61 % Al₂O₃, 26 to 35% TiO₂, and 0.5 to 5% Fe₂O₃; and,

the filter has a porosity of 40% to 50% by volume, a median pore size of 10 to 15 micrometers, and a permeability of 0.33 to 1.00 x 10^-12 m².

3. The diesel exhaust particulate filter of claim 2 wherein the ceramic contains, expressed in terms of weight percent of the total body, of 70% iron-aluminum titanate solid solution having a stoichiometry of Al_2(1-X)Fe_2XTiO₅, where x is 0-0.1, and 30% mullite (3Al₂O₃·2SiO₂).

4. The diesel exhaust particulate filter of claim 1 further having a CTE, as measured from room temperature to 1000°C of 5 to 15 x 10^-7/°C, a heat capacity of at least 3.2 J cm^-3K^-1 at 500°C, and a four-point modulus of rupture as measured on a solid rod of circular cross section of at least 34 bar (500 pounds per square inch).

5. The diesel exhaust particulate filter of claim 4 wherein the CTE, as measured from room temperature to 1000°C is 5 to 10 x 10^-7/°C, a heat capacity of at least 3.9 J cm^-3K^-1 at 500°C, and a four-point modulus of rupture as measured on a solid rod of circular cross section of at least 55 bar (800 pounds per square inch).

6. A method of making a structure for use in filtration of diesel exhaust, the method comprising:

a. forming a mixture of powdered raw materials selected to obtain a ceramic article consisting essentially, expressed in terms of weigh percent on the oxide basis, of 3 to 15% SiO₂, 55 to 65% Al₂O₃, 22 to 40% TiO₂, and 0 to 10% Fe₂O₃;

b. shaping the mixture into a green ceramic article;

c. firing the green ceramic article to a temperature of between 1650° to 1700°C for a duration of time sufficient to produce a mullite-aluminum titanate ceramic article having a porosity of 35 % to 55 % by volume, a median pore size of 8 to 20 micrometers, a permeability of at least 0.30 x 10^-12 m², and, an increment in a linear dimension of less than 1% after the filter has been exposed one hundred times to thermal cycling at a temperature of 200° to 1100°C in a time of 7 minutes, and cooling from 1100° to 200°C in a time of 7 minutes.

7. The method of claim 6 wherein the raw materials comprise oxide sources including:

silica (SiO₂) having a particle size of 5 to 20 micrometers;

alumina (Al₂O₃) having a particle size of 5 to 25 micrometers;

titania (TiO₂) having a particle size of 4 to 20 micrometers; and,

iron oxide (Fe₂O₃) added at about 0 to 6% weight of the total batched raw materials.

8. The method of claim 7 wherein:

the silica (SiO₂) has a particle size of 8 to 15 micrometers;

alumina (Al₂O₃) having a particle size of 10 to 15 micrometers;

titania (TiO₂) having a particle size of 8 to 16 micrometers; and,

iron oxide (Fe₂O₃) added at about 0 to 5% weight of the total batched raw materials.

9. The method of claim 6 wherein the mixture is shaped by extrusion into a honeycomb having an inlet and outlet end and a multiplicity of cells extending from inlet end to outlet end, the cells having porous walls, wherein a portion of the cells at the inlet end and a portion of the cells at the outlet end are plugged.

10. The method of claim 9 wherein the firing step includes heating from room temperature (22-25°C) to 1500°-1600°C over a period of about 8 to 10 hours, with a hold at top temperature of about 4 to 8 hours, followed by heating over a period of about 2 to 3 hours to a top temperature of 1650° to 1700°C, with a hold thereat of about 30 to 60 minutes, followed by a ramp down to a temperature of about 1575° to 1600°C over a period of about 1 hour, with a hold thereat of about 4 to 5 hours.

Ansprüche

1. Dieselabgaspartikelfilter umfassend einen mit Pfropfen versehenen Wandfluss-Honigwabenfilterkörper, der aus porösem Keramikmaterial besteht und mehrere parallele, mit Endpfropfen versehene Zellkanäle umfasst, die den Körper von einem Vordereinlassende bis zu einem Auslassende desselben durchqueren, wobei:

das Keramikmaterial, in Gewichtsprozent des gesamten Körpers ausgedrückt, eine aus 60 - 90 % Eisen-Aluminium-Titanat bestehende feste Lösung mit einer Stöchiometrie von Al_2(1-x)Fe_2xTiO₅, wobei x 0 - 0,1 beträgt, und 10 - 40 % Mullit (3Al₂O₃·2SiO₂) enthält und im Wesentlichen, als Gewichtsprozent auf das Oxid bezogen, aus 3 bis 15 % SiO₂, 55 bis 65 % Al₂O₃, 22 bis 40 % TiO₂ und 0 bis 10 % Fe₂O₃ besteht; und

der Filter eine Porosität von 35 bis 55 Vol.-%, eine mittlere Porengröße von 8 bis 20 Mikrometern, eine Durchlässigkeit von mindestens 0,30 x 10^-12 m², wenn das Brennen bei einer Temperatur zwischen 1650 bis 1700 °C erfolgt, und eine Vergrößerung in linearer Dimension von weniger als 1 %, nachdem der Filter einhundertmal dem Wärmezyklieren bei einer Temperatur von 200 bis 1100 °C innerhalb einer Zeitspanne von 7 Minuten, und Kühlen von 1100 auf 200 °C innerhalb einer Zeitspanne von 7 Minuten ausgesetzt worden ist, aufweist.

2. Dieselabgaspartikelfilter nach Anspruch 1, wobei:

das Keramikmaterial in Gewichtsprozent des gesamten Körpers ausgedrückt, eine aus 70 - 80 % Eisen-Aluminium-Titanat bestehende feste Lösung mit einer Stöchiometrie von Al_2(1-x)Fe_2xTiO₅, wobei x 0 - 0,1 beträgt, und 20 - 30 % Mullit (3Al₂O₃·2SiO₂) enthält und im Wesentlichen, als Gewichtsprozent auf das Oxid bezogen, aus 6 bis 12 % SiO₂, 57 bis 61 %Al₂O₃, 26 bis 35 % TiO₂ und 0,5 bis 5 % Fe₂O₃ besteht; und

der Filter eine Porosität von 40 bis 50 Vol.-%, eine mittlere Porengröße von 10 bis 15 Mikrometern und eine Durchlässigkeit von 0,33 bis 1,00 x 10^-12 m² aufweist.

3. Dieselabgaspartikelfilter nach Anspruch 2, wobei das Keramikmaterial in Gewichtsprozent des gesamten Körpers ausgedrückt, eine aus 70 % Eisen-Aluminium-Titanat bestehende feste Lösung mit einer Stöchiometrie von Al_2(1-x)Fe_2xTiO₅ enthält, wobei x 0 - 0,1 beträgt, und 30 % Mullit (3Al₂O₃·2SiO₂) enthält.

4. Dieselabgaspartikelfilter nach Anspruch 1, des Weiteren einen WAK, von der Raumtemperatur bis 1000 °C gemessen, von 5 bis 15 x 10^-7/°C, eine Wärmekapazität von mindestens 3,2 Jcm^-3K^-1 bei 500 °C und einen Vierstellenbruchmodul, wie an einem massiven Stab von rundem Querschnitt gemessen, von mindestens 34 bar (500 Pfund pro Quadratzoll) aufweist.

5. Dieselabgaspartikelfilter nach Anspruch 4, wobei der WAK, von der Raumtemperatur bis 1000 °C gemessen, 5 bis 10 x 10^-7/°C beträgt, eine Wärmekapazität von mindestens 3,9 Jcm^-3K^-1 bei 500 °C und einen Vierstellenbruchmodul, wie an einem massiven Stab von rundem Querschnitt gemessen, von mindestens 55 bar (800 Pfund pro Quadratzoll) aufweist.

6. Verfahren zum Herstellen eines Bauteils zur Verwendung beim Filtrieren von Dieselabgas, wobei das Verfahren Folgendes umfasst:

a) das Bilden einer Mischung von pulverförmigen Rohmaterialien, ausgewählt, um einen Keramikgegenstand bestehend im Wesentlichen, in Gewichtsprozent auf das Oxid bezogen ausgedrückt, von 3 bis 15 % SiO₂, 55 bis 65 % Al₂O₃, 22 bis 40 % TiO₂ und 0 bis 10 % Fe₂O₃ zu erhalten;

b) Formen der Mischung zu einem grünen Keramikartikel ;

c) Brennen des grünen Keramikartikels auf eine Temperatur zwischen 1650 bis 1700 °C für eine Zeitspanne, die ausreicht, um einen Mullit-Aluminium-Titanat-Keramikartikel mit einer Porosität von 35 % bis 55 Vol.-% , einer mittleren Porengröße von 8 bis 20 Mikrometern, einer Durchlässigkeit von mindestens 0,30 x 10^-12 m² und einer Erhöhung in linearer Dimension von weniger als 1 %, nachdem der Filter einhundertmal dem Wärmezyklieren bei einer Temperatur von 200 bis 1100 °C innerhalb einer Zeitspanne von 7 Minuten und Kühlen von 1100 auf 200 °C innerhalb einer Zeitspanne von 7 Minuten ausgesetzt worden ist, aufweist

7. Verfahren nach Anspruch 6, wobei die Rohmaterialien Oxidquellen umfassen, einschließlich:

Siliciumdioxid (SiO₂) mit einer Teilchengröße von 5 bis 20 Mikrometer;

Aluminiumoxid (Al₂O₃) mit einer Teilchengröße von 5 bis 25 Mikrometer;

Titandioxid (TiO₂) mit einer Teilchengröße von 4 bis 20 Mikrometer; und

Eisenoxid (Fe₂O₃), das in einer Menge von etwas 0 bis 6 Gew.-%, auf das gesamte chargenförmige Material bezogen, zugegeben wird.

8. Verfahren nach Anspruch 7, wobei:

das Siliciumdioxid (SiO₂) eine Teilchengröße von 8 bis 15 Mikrometern aufweist;

das Aluminiumoxid (Al₂O₃) eine Teilchengröße von 10 bis 15 Mikrometern aufweist;

das Titandioxid (TiO₂) eine Teilchengröße von 8 bis 16 Mikrometern aufweist;
und

das Eisenoxid (Fe₂O₃) einer Menge von etwas 0 bis 5 Gew.-%, auf das gesamte chargenförmige Material bezogen, zugegeben wird.

9. Verfahren nach Anspruch 6, wobei die Mischung durch Extrusion in eine Honigwabe mit einem Einlass- und einem Auslassende und mehreren Zellen, die sich von dem Einlassende bis zum Auslassende erstrecken, geformt wird, wobei die Zellen poröse Wände aufweisen, wobei ein Teil der Zellen am Einlassende und ein Teil der Zellen am Auslassende mit Pfropfen versehen sind.

10. Verfahren nach Anspruch 9, wobei der Brennschritt das Erhitzen von Raumtemperatur (22 - 25 °C) auf 1500 - 1600 °C über eine Zeitspanne von etwa 8 bis 10 Stunden mit Halten bei der Spitzentemperatur für etwa 4 bis 8 Stunden, gefolgt vom Erhitzen über eine Zeitspanne von etwa 2 bis 3 Stunden auf eine Spitzentemperatur von 1650 bis 1700 °C mit Halten dabei für etwa 30 bis 60 Minuten, gefolgt von einem Absenken auf eine Temperatur von etwa 1575 bis 1600 °C über eine Zeitspanne von etwa 1 Stunde, mit Halten dabei für etwa 4 bis 5 Stunden, umfasst.

Revendications

1. Filtre particulaire de gaz d'échappement diesel comprenant un corps de filtre obturé présentant une structure en nid d'abeilles écoulement-paroi, composé d'un matériau céramique poreux et comprenant une pluralité de canaux parallèles à cellules obturées en bout, traversant le corps depuis une extrémité d'entrée frontale vers une extrémité de sortie de ce dernier, dans lequel :

la céramique contient, exprimé en termes de pourcentage en poids du corps total, 60-90 % de solution solide de titanate de fer-aluminium ayant une stoechiométrie de Al_2(1-x)Fe_2xTiO₅, où x est 0-0,1, et 10 - 40 % de mullite (3Al₂O₃-2SiO₂) et se compose essentiellement, exprimé en termes de pourcentages en poids, sur la base d'oxydes, de 3 à 15 % de SiO₂, de 55 à 65% de Al₂O₃, de 22 à 40 % de TiO₂ et de 0 à 10 % de Fe₂O₃ ; et,

le filtre a une porosité de 35 % à 55 % en volume, une taille moyenne de pores de 8 à 20 micromètres, une perméabilité d'au moins 0,30 x 10^-12 m² en cas de cuisson à une température comprise entre 1650 °C et 1700 °C, et un incrément dans une dimension linéaire de moins de 15 % après une exposition du filtre à cent reprises à un cyclage thermique à une température de 200 °C à 1100°C dans une période de 7 minutes, et après un refroidissement allant de 1100 °C à 200 °C dans une période de 7 minutes.

2. Filtre particulaire de gaz d'échappement diesel selon la revendication 1, dans lequel :

la céramique contient, exprimé en termes de pourcentage en poids du corps total, 70-80 % de solution solide de titanate de fer-aluminium ayant une stoechiométrie de Al_2(1-x)Fe_2xTiO₅, où x est 0-0,1, et 20-30 % de mullite (3Al₂O₃-2SiO₂) et se compose essentiellement, exprimé en termes de pourcentages en poids, sur la base d'oxydes, de 6 à 12 % de SiO₂, de 57 à 61% de Al₂O₃, de 26 à 35 % de TiO₂ et de 0,5 à 5 % de Fe₂O₃ ; et,

le filtre a une porosité de 40 % à 50 % en volume, une taille moyenne de pores de 10 à 15 micromètres et une perméabilité de 0,33 à 1,00 x 10^-12 m².

3. Filtre particulaire de gaz d'échappement diesel selon la revendication 2, dans lequel la céramique contient, exprimé en pourcentage en poids du corps total, 70% de solution solide de titanate de fer-aluminium ayant une stoechiométrie Al_2(1-x)Fe_2xTiO₅, où x est 0-0,1, et 30 % de mullite (3Al₂O₃-2SiO₂).

4. Filtre particulaire de gaz d'échappement diesel selon la revendication 1, ayant en outre une valeur CTE, telle que mesurée de la température ambiante à la température de 1000 °C, de 5 à 15 x 10^-7/°C, une capacité calorifique d' au moins 3,2 J cm^-3 K^-1 à la température de 500 °C et un module de rupture à quatre points, tel que mesuré sur une tige solide de section transversale circulaire, d'au moins 34 bars (500 livres par pouce au carré).

5. Filtre particulaire de gaz d'échappement diesel selon la revendication 4, dans lequel la valeur CTE, telle que mesurée de la température ambiante à la température de 1000 °C, est de 5 à 10 x 10^-7/°C, une capacité calorifique d'au moins 3, 9 J cm^-3K^-1 à la température de 500 °C et un module de rupture à quatre points, tel que mesuré sur une tige solide de section transversale circulaire, d'au moins 55 bars (800 livres par pouce au carré).

6. Procédé de fabrication d'une structure en vue d'une utilisation dans la filtration de gaz d'échappement diesel, le procédé comprenant :

a) la formation d'un mélange de matières brutes pulvérulentes sélectionnées pour obtenir un article céramique se composant essentiellement, exprimé en termes de pourcentage en poids, sur la base des oxydes, de 3 à 15 % de SiO₂, de 55 à 65 % de Al₂O₃, de 22 à 40 % de TiO₂ et de 0 à 10 % de Fe₂O₃ ;

b) la moulage du mélange pour former un article céramique vert ;

c) la cuisson de l'article céramique vert à une température comprise entre 1650 °C à 1700 °C pendant une période de temps suffisante afin de produire un article céramique en mullite-titanate d'aluminium, ayant une porosité de 35 % à 55 % en volume, une taille moyenne de pores de 8 à 20 micromètres, une perméabilité d'au moins 0,30 x 10^-12 m², et un incrément dans une dimension linéaire de moins de 1 % après une exposition du filtre à cent reprises à un cyclage thermique à une température de 200 °C à 1100 °C dans une période de 7 minutes, et après un refroidissement allant de 1100 °C à 200 °C dans une période de 7 minutes.

7. Procédé selon la revendication 6, dans lequel les matières brutes comprennent des sources d'oxydes comprenant :

de la silice (SiO₂) ayant une taille de particule de 5 à 20 micromètres ;

de l'alumine (Al₂O₃) ayant une taille de particule de 5 à 25 micromètres ;

de l'oxyde de titane (TiO₂) ayant une taille de particule de 4 à 20 micromètres ; et,

de l'oxyde de fer (Fe₂O₃) ajouté à raison d'environ 0 à 6 % en poids des matières brutes totales mises sous forme de lots.

8. Procédé selon la revendication 7, dans lequel :

la silice (SiO₂) a une taille de particule de 8 à 15 micromètres ;

l'alumine (Al₂O₃) a une taille de particule de 10 à 15 micromètres ;

l'oxyde de titane (TiO₂) a une taille de particule de 8 à 16 micromètres; et,

l'oxyde de fer (Fe₂O₃) est ajouté à raison d'environ 0 à 5 % en poids des matières brutes totales mises sous forme de lots.

9. Procédé selon la revendication 6, dans lequel le mélange est mis en forme par extrusion dans une structure en nid d'abeilles ayant une extrémité d'entrée et de sortie et une multiplicité de cellules se prolongeant de l'extrémité d'entrée à l'extrémité de sortie, les cellules ayant des parois poreuses, une portion des cellules à l'extrémité d'entrée et une portion des cellules à l'extrémité de sortie étant obturées.

10. Procédé selon la revendication 9, dans lequel l'étape de cuisson comprend le chauffage allant de la température ambiante (22-25 °C) à 1500-1600 °C en l'espace d'une période d'environ 8 à 10 heures, avec un maintien à la température haute pendant environ 4 à 8 heures, opération suivie par un chauffage en l'espace d'une période d'environ 2 à 3 heures à une température haute de 1650 °C à 1700 °C, avec un maintien à cette température pendant environ 30 à 60 minutes, opération suivie d'une rampe de descente à la température d'environ 1575 °C à 1600 °C en l'espace d'une période d'environ 1 heure, avec un maintien à cette température pendant environ 4 à 5 heures.

Drawing